[llvm-bugs] [Bug 49991] New: Question on backporting DR fixes

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49991

            Bug ID: 49991
           Summary: Question on backporting DR fixes
           Product: clang
           Version: trunk
          Hardware: Other
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: cpp at daryl.haresign.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

I spent some time today investigating why Clang was rejecting the following
code in C++17 mode, but not C++20. MSVC and GCC both accept it in C++17.

#include <optional>
struct C {
  C(C&) = default;
};
std::optional<C> o;

It seems https://wg21.link/cwg1331 was addressed by https://wg21.link/p641,
which was accepted into C++20.

Clang has the following commit which applies the change only in C++20 mode:
https://github.com/llvm/llvm-project/commit/9b2c5e7c44fd28c31f4a0b088d0bf71da4fddfa5

Whereas GCC and MSVC seem to have applied it unconditionally.

Is there any policy for when defect fixes are applied in earlier standard
modes?

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